There are many applications for high power, eye-safe lasers in communications, industrial material processing, remote-sensing and medical/dental surgery. In U.S. Pat. Nos. 5,299,210 and 5,566,196, clad-pumped fiber lasers use rare earth ions as the lasing medium, at wavelengths below 1.5 .mu.m. However, fiber lasers emitting wavelengths above 1.5 .mu.m are limited to less than a few milliwats. Additionally, with Er and Ho ions, which have eye-safe wavelengths above 1.5 .mu.m, lasing occurs between a first excited state and the ground state; therefore, lasing cannot be sustained unless the cavity loss due to the ground state absorption at the lasing wavelength is offset by the gain of the system.
It has been shown that the ground state Er ions can be energized indirectly by utilizing Ytterbium (Yb) as a stimulating codopant, with an appropriate concentration ratio (up to about 12:1) in comparison with the concentration of Er. The higher the codoping ratio, the greater the absorption and therefore the shorter the length of fiber. See, for instance, Townsend et al, YB.sup.3+ Sensitized ER.sup.3+ doped Silica Optical Fiber With Ultrahigh Frequency and Gain, Elec. Lett., Oct. 10, 1991, pp. 1968-1969, and Kringlebotn et al, Highly Efficient, Low-noise Grating-feedback ER.sup.3+ : YB.sup.3+ Codoped Fiber Laser, Elect. Lett., Jun. 9, 1994, pp. 972-973. While codoping may increase the effective pumping rate of the ground state of ER.sup.3+ to the upper laser level--by as much as fifty times--the enhancement of pumping decreases rapidly with increasing pumping power. See, for instance, Ding and Cheo, Effects of Yb: Er-Codoping on Suppressing Self-Pulsing In Er-Doped Fiber Lasers, IEEE Photon. Tech. Lett. Mar. 3, 1997, pp. 324-326. For these reasons, high power fiber lasers emitting at eye-safe wavelengths have heretofore not been successful.
Attempts to increase clad pumped fiber laser power have included the use of multiple cores. However, the output power is usually either incoherent or in multiple beam spots (commonly referred to as "higher-order supermodes").
Another problem with clad-pumped fiber lasers is that the same energy that eventually is to be absorbed in the core so as to pump the lasing medium is traveling a great distance within the cladding, which can also absorb that energy significantly as the length of the fiber becomes too long. Therefore, while increasing the length of the laser fiber will increase the exposure of the core to the energy to be absorbed by the core and therefore increase the transfer efficiency, it also increases the amount of energy absorbed in the cladding, by some efficiency ratio. Thus, although lasers formed in coils of lengths in the kilometer regime have been known, they are not very efficient.
In the prior art of clad-pumped lasers, it is common to use a single mirror at the pump-input end of the fiber, and use of a second mirror or a cleaved fiber end as a mirror at the output end, if any, is confined to reflection of the pump laser wavelength, being nearly invisible to the laser output wavelengths.